1. Field of the Invention
The present invention relates to a driving voltage controller for a light-emitting source, which controls a driving voltage that drives and lights a light-emitting source, and more particularly, relates to a driving voltage controller for a light-emitting source, which is used for preventing the luminance flicker of the light-emitting source caused by a change in the power supply voltage, by performing the PWM (pulse-width modulation) control of the driving voltage.
2. Description of Related Art
In general, a system, which supplies electric power to a light-emitting source from a power supply, and which further supplies power to an electric power load other than the light-emitting source from the same power supply, is known. For example, in a motor vehicle, light-emitting sources and motors are driven with one battery serving as a power supply.
However, there is a trend toward more and more electrical appliances such as an air-conditioner, a navigation system, and an audio system, which are electric power-supply loads, in a vehicle. When electric power is supplied from one battery to power-supply loads such as light-emitting sources and various electrical systems, there occurs a problem that the battery voltage (power supply voltage) changes caused by the turning-on-or-off of the power to the power-supply loads. This change of the power supply voltage causes a luminance-flickering phenomenon in the light-emitting sources.
Known methods for preventing such flicker include a method of PWM-controlling the driving voltage applied to an illumination lamp or a light-emitting source. For example, a battery power source is equipped with a voltage regulator in which a resistor and a Zener diode are connected in series.
Further, for example, JP-A-11-233276 (in Paragraphs [0012] to [0016] and FIG. 1) discloses an illumination adjusting method by which the illumination of an illumination lamp or a light-emitting source is adjusted by the following method: a switching transistor is added to the above voltage regulator; the positive side DC voltage of an electrolytic capacitor, which is obtained by applying a PWM signal to the base of the switching transistor to convert the PWM signal into the voltage, is applied to the base of a driving transistor; power is supplied from a power supply to the light-emitting source by use of this driving transistor; and thereby the DC voltage impressed to the base of the driving transistor is controlled with respect to voltage by changing the pulse width of the PWM signal.
Moreover, JP-A-2003-338396 (in Paragraphs [0017] to [0035] and FIGS. 1-3) discloses a method of preventing the flicker of a light-emitting source, characterized in that, when the light-emitting source is driven and lit by means of PWM control by use of a power supply voltage, the duty ratio of a PWM signal is changed according to the fluctuation of the power supply voltage, and the frequency of the PWM signal at the start of driving power-supply loads other than the light-emitting source is controlled such that the frequency thereof is higher than that of the PWM signal at the stationary time.
Here, when controlling the duty ratio of the PWM signal according to the fluctuation of the power supply voltage, the higher is the frequency of the PWM signal, the easier is to PWM-control the light-emitting source at the optimum duty ratio according to the fluctuation of the power supply voltage. However, constantly setting the frequency of the PWM signal at a higher value not only increases the heat generation from the high-speed switch, but also causes the increase of noise. For this reason, the PWM signal is controlled such that the frequency thereof becomes higher only at the startup time of the other power-supply loads.
Further, by this flicker-preventing method, when the power supply voltage V returns to a stable state after having been momentarily greatly changed at the startup time of the other power-supply loads, the duty ratio of the PWM control is changed at high speed to thereby perform the optimum PWM control of the light-emitting source, while at the stable state of the power supply voltage, the frequency of the PWM signal is comparatively reduced such that the occurrence of the problem of the heat generation by the high-speed switch is prevented.
The conventional driving voltage controller of a light-emitting source is arranged as mentioned above. That is, the driving voltage controller of the light-emitting source is arranged such that the illumination of the illumination lamp or the light-emitting source is adjusted by: changing the pulse width of the PWM signal; applying the positive side DC voltage of the electrolytic capacitor, which is obtained by converting the PWM signal into the voltage, to the base of the driving transistor; and controlling the DC voltage applied to the base of the driving transistor with respect to voltage. For this reason, there is a problem that the amount of heat generated by the driving transistor to which the DC voltage is applied becomes large.
In addition, the conventional driving voltage controller of the light-emitting source must be equipped with various circuit elements. As a result, there is a problem that not only the scale of the circuit of the controller must be large, but also the driving transistor itself must be expensive for coping with the heat generation.
Moreover, in the conventional driving voltage controller of the light-emitting source, there is a problem that, when the power supply voltage repeats the rapid change, the luminance of the light-emitting source actually changes for responding to the change of the voltage, resulting in the increase of the flicker, which is visually perceived by the user.
Additionally, in the conventional driving voltage controller of the light-emitting source, it is arranged that the duty ratio of the PWM signal be changed according to the fluctuation of the power supply voltage, and the frequency of the PWM signal at the start of driving the power-supply loads other than the light-emitting source be controlled such that the frequency thereof is higher than that of the PWM signal at the stationary time. Therefore, when the power supply voltage returned to its original stationary state after the voltage rapidly dropped, the driving voltage of the light-emitting source also rapidly returns to its original stationary state in response thereto. For this reason, there is a problem that the light-emitting source of which luminance once reduced rapidly increases its luminance, which causes the flicker thereof to feel like as if it is increased.